Implanted medical devices often include elongate medical electrical leads carrying at least one conductor electrically coupled to a tissue-contacting electrode; examples of such devices include, but are not limited to, pacemakers, cardioverter-defibrillators and neurostimulators. It is known in the art that medical electrical leads can act as antennas, which ‘pick up’ electromagnetic radiation of radio-frequency (RF) pulses, for example used for Magnetic Resonance Imaging (MRI), and that these pulses can induce a appreciable amount of current in lead conductors. When this current is induced in a lead conductor, a relatively large current gradient can develop across the resistive interface between the electrode of that conductor and the tissue interfacing with the electrode; such a gradient causes heat to be released into the tissue, which may be injurious to the tissue.
It is further known in the art to insert, in series, an electromagnetic trap, in the form of an inductor element, between the conductor of a lead and the tissue-contacting electrode. The inductor element acts as a high-frequency resistor attenuating RF pulse-induced current flowing to the electrode and thereby reduces undesirable tissue heating. Although such inductor elements have been proposed for medical electrical leads, there is still a need for practical designs which integrate such an element into medical leads without compromising other lead features, for example, pertaining to ease of implantation, stable fixation and long term performance of the lead.
Many medical electrical leads include elongate conductor coils coupled to tissue-contacting electrodes. In some cases, these conductor coils may be designed to have inductive properties that effectively reduce the undesirable tissue heating described above. However, in other cases, for example, when the conductor coil must be stiff enough to transfer torque in order to steer a lead to, and/or fix a lead electrode at, a target site, it may not be possible for the coil to have the inductive properties that reduce tissue heating.